1. Field of the Invention
The present invention relates to an image heating apparatus preferably for use as a fixing device in a copying machine, a laser beam printer or the like, and particularly to an image heating apparatus having a plurality of heat generating elements therein.
2. Related Background Art
Conventionally a thermal fixing apparatus contained in this type of image forming apparatus fixes an unfixed image (toner image) formed on a recording sheet by an electrophotographic process or other image forming means into the recording sheet, and there are well known types such as a thermal-roller type fixing apparatus having a halogen heater as a heat source or a film-heating type thermal fixing apparatus having a ceramic flexible heating sheet heater as a heat source.
FIG. 8 shows an example of a general heater driver circuit adopted to this type of thermal fixing apparatus.
As shown in FIG. 8, a heater 803 generally comprising a plurality of heat generating elements is connected to a commercial AC power supply 801 via a triac or other switching control elements 804 and 816 and power is supplied from this AC power supply 801. The heater 803 is provided with a temperature detecting element, for example, a thermistor 814, a temperature of the heater 803 is detected by the temperature detecting element 814, a control circuit (power supply instruction means) 812 is turns on or off the switching control elements 804 and 816, by which a power supply to the heater 803 is turned on or off to control a temperature of the thermal fixing apparatus to a certain temperature of a target.
The on or off control of the power supply to the heater 803 is performed by a wave number control or a phase control of the commercial power supply 801.
The heater 803 has two generating elements, each having a length according to a width of a recording sheet, and therefore two heat generating elements are not concurrently energized. Filters 811 and 823 are provided to remove switching noises generated from the switching control elements 804 and 816 by turning on or off the heater 803.
The conventional apparatus set forth in the above requires switching control elements for controlling the heater by the number of the heat generating elements of the heater. In this condition, the switching control elements 804 and 816 for supplying power must turn on or off a power supply for large current to the heater 803, thereby causing a need for being increased in size so as to cope with energizing of large current for driving the heater. This increase in size of the elements causes an increase of an amount of heat generated from the elements at switching or an increase of noises generated by the switching operation. Therefore, it is further required to take countermeasures against heat generation caused by the switching operation or to provide a filter for absorbing the noises.
In view of these problems, the present invention has been provided, and therefore it is an object of the present invention to provide an image heating apparatus requiring only a small number of semiconductor switching elements in spite of having a plurality of heat generating elements.
It is another object of the present invention to provide an image heating apparatus, comprising:
a heating member having a first heat generating element and a second heat generating element;
relay means for relaying between a power supply and said heating member, said relay means connecting either said first heat generating element or said second heat generating element to the power supply;
switching means arranged between the power supply and said heating member; and
control means for controlling said switching means so that a temperature of said heating member is maintained at a set temperature.